1. Filed of the Invention
The present invention relates to a novel method of overwriting an information on a recording material by the use of a laser beam of high-energy density.
2. Description of Prior Art
Information recording media utilizing a beam of high energy density such as a laser beam have been developed in recent years and now are put to practical use. Such recording medium is called an optical disc and its practical applications have been found, for example, as a video disc and an audio disc as well as a disc memory for a large-capacity computer, a large-capacity static image file, an optical card, a micro-image recording medium, a supermicro-image recording medium, a micro-facsimile and a negative for photographic typesetting.
The optical disc basically comprises a disc-shaped transparent substrate of plastic or glass material and a recording layer provided on the substrate. As materials of the recording layer, that is, recording materials, there are known a metal such as Bi, Sn, In or Te, a semimetal, and a dye such as cyanine dye, metal complex dye or quinone dye.
Writing (or Recording) of information on the optical disc is conducted, for example, by irradiating a rotating optical disc with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs energy of the beam and a rise in temperature locally occurs, and as a result chemical or physical change (such as formation of a pit) is caused to alter (or change) optical characteristics of the recording layer in the irradiated area, whereby the information can be recorded. The alteration of optical characteristics can be done through formation of pits or protruded portions on the surface of the recording layer or formation of bubbles in the recording layer. The recording of information utilizing the formation of pits is widely adopted. Further, a method of utilizing a reaction between two materials in the recording layer is also used.
Reading (or reproduction) of information from the optical disc is conducted by irradiating the optical disc with a laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the local change of the optical characteristics of the recording material.
Generally, the above-mentioned system for recording and reproducing an information cannot be repeated in one recording medium. In other words, an information recorded in the recording medium can be reproduced repeatedly but cannot be erased for recording of another information thereon. This is because the recording material of such recording medium cannot be converted reversibly.
Erasure of information for another recording (i.e., overwriting) on the optical disc can be done only in the case that the information is recorded through reversible change of the recording material. Such recording medium is named an erasable recording medium. In the medium of this type, information is generally recorded, erased and recorded through reversible change of the phase condition (i.e., phase change), such as change between a crystalline phase and an amorphous phase, change between a homogeneous clear (transparent) phase and a phase-separated turbid phase, or change between a regularly oriented crystalline phase and an irregularly oriented crystalline phase.
Overwriting of information in the erasable recording medium can be done by a two stage procedure or a one stage procedure. The former two stage procedure comprises a separated two stages, that is, a stage for erasing a recorded information from the recording medium and a stage for recording (writing) a fresh information on the recording medium having been subjected to the above erasing stage. The latter one stage procedure comprises an erasing operation and a recording operation which is inserte in the erasing operation at appropriate times.
Recording of information on an optical disc according to a known two stage procedure is done, for instance, by initially heating the recording layer to crystallize the whole surface of the layer and then irradiating the crystalline recording layer of the optical disc under rotation intermittently with a laser beam of high power to form on the recording layer a series of plural separated amorphous areas corresponding to digital signals of the information. Thus, the information is recored on the recording layer. Erasure of the recorded information is done by irradiating areas including the amorphous areas of the recording layer of the disc under rotation continuously with a laser beam having a thermal energy lower than that of the laser beam used in the recording procedure to convert the amorphous phase areas into crystalline phase areas. Thus, the information-erased recording layer of the optical disc now has a crystalline phase in its whole area. Recording of another (fresh) information on the erased recording layer can be done in the manner as described above for the first recording.
The above-described two-stage overwriting procedure is disadvantageous in that satisfactory erasure is sometimes not achieved in the erasing operation. This is explained in detail by referring to FIGS. 1 and 2 in the attached drawings. FIG. 1 is a shematic view of a recording layer having received irradiation of a laser beam in the recording operation.
When the recording layer is irradiated with a laser beam of a high power in the recording operation, the recording material in the irradiated area 1 (i.e., irradiated spot) absorbs high thermal energy of the laser beam to melt instantly. Subsequently the irradiation of laser beam is stopped and then the irradiation is again applied to an area 1' which is arranged ajacently to the melted area 1 with a certain space. In the course of the above procedure, the melted area 1 is rapidly cooled just after the irradiation is stopped, and turns not to the original stable crystalline phase area but to a metastable amorphous phase area. By the formation of a series of the metastable amorphous phase areas, the desired information is recorded in a set of digital siganals. It has been noted that a recrystallized area 2 is incidentally produced around the formed amorphous area (recorded area) 1 because the area 2 around the recorded area 1 is also heated up to a temperature below the melting point of the recording material of the layer but above the recrystallization temperature (or phase-transition temperature). The condition of the recrystallized area 2 is different from that of the crystalline background area 3. Accordingly, when the recording operation is complete, the recording layer has the background crystalline area 3 and the information-recorded amorphous area 1, as well as another crystalline area 2 which differs from the background crystalline area S in its crystalline conditions.
FIG. 2 is a schematic view of the recording layer of FIG. 1 after being subjected to the erasing operation.
In the erasing operation, the recording layer is irradiated with a laser beam of a relatively low power to continuously scan to trace a route which is so predetermined as to connect the information-recorded amorphous areas. The continuous belt 4 of FIG. 2 shows a track of spots of the scanned laser beam. After the erasing operation using the low power laser beam is complete, the area of belt 4 turns into the orginal cystalline phase area but the areas 2a, 2b are left having the different crystalline condition. Therefore, thus erased recording layer has two kinds of different crystalline areas which are slightly different in the optical characteristics. The presence of such different phase areas in the information erased recording layer sometimes causes error or failure of the following recording, reproduction or other operations.
To solve the above-mentioned problem, a one-stage overwriting procedure using a single laser beam has been proposed. According to the procedure, the erasure is done by continuously applying a laser beam of high power onto an information-recorded recording layer. In the course of this operation, the recording material in the amorphous phase area as well as in the two kinds of the crystalline phase areas is similarly melted to turn into a homogeneous crystalline phase area. When the laser beam reaches a position where a new information is to be recorded, the power of the laser beam is instantly lowered to make the recording material at the position amorphous. In this manner, the one-stage overwriting (combination of erasure of a recorded information and subsequent recordal of another information) can be attained using a single laser beam. According to this method, the problem of production of different crystalline phase area is obviated. However, since the recording layer has to be continuously irradiated with the laser beam of high power, irreversible change is apt to occur in the recording layer due to deterioration of the recording material. The occurrence of such irreversible change may cause increase of read-out error after the overwriting procedure is repeatedly performed.
As another method, a pseudo-overwriting method has been proposed. This method uses two laser beams simultaneously, in which erasure is done with an elliptic beam and writing is done with a circular beam. There are disadvantages that a conventional optical head of writeonly-type cannot be used due to necessity of a set of two beams and that adjustment such as positional adjustment between two beams is not easy.